Spectrophotometric Determination of Lanthanum in Plutonium KARL S. BERGSTRESSER University o f California, los Alamos Scientific Laboratory, l os Alamos,
b Lanthanum in plutonium samples can b e determined by formation of plutonium(V1) in a perchloric acid solution of the sample and precipitation of lanthanum with hydrofluoric acid. The fluoride precipitate is dissolved in fuming perchloric acid, and lanthanum is reprecipitated with 8-quinolinol. The lanthanum oxinate is dissolved in 0.1N hydrochloric acid to obtain a solution which is suitable for photometric measurements at 365 mp. The standard deviation for a single measurement is equivalent to 3 y for a range of 40 to 200 y of lanthanum.
A
method has been used in this laboratory for the determination of lanthanum in plutonium metal (5) and solutions (4). The method has a range of 50 to 10,000 p.p.m. of lanthanum with respect to plutonium and a standard deviation of approximately 20%. For some plutonium-lanthanum samples, particularly those containing 500 to 20,000 p.p.m. of lanthanum, the reliability of the spectrographic method is not always adequate. Consequently, the possibility of determining lanthanum in plutoniumcontaining samples by other methods was investigated. It was recognized that a spectrophotometric method would require an initial separation of microgram amounts of lanthanum from milligram quantities of plutonium, as methods for determining lanthanum are not specific. Sandell (6) describes two colorimetric methods for lanthanum: formation of a lake with alizarin, and an indirect determination in which lanthanum is precipitated as the phosphate and the phosphorus in the dissolved precipitate is determined by the molybdenum blue method. In both of these methods many other metals, including plutonium, will react and interfere. During the search for a separation method, it v a s recalled that quantitative precipitation of lanthanum had been used in plutonium radiochemical work continuously from the beginning of the Plutonium Project (2). Khen lanthanum is added to plutonium solutions and is precipitated with hydrofluoric SPECTROGRAPHIC
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ANALYTICAL CHEMISTRY
N. M.
acid, the lanthanum fluoride serves as a carrier for plutonium(II1) and (IV), but plutonium(V1) does not coprecip itate under these conditions. If a plutonium-lanthanum solution is treated with a suitable oxidizing agent, and only sexivalent plutonium is present during the formation of lanthanum fluoride, a separation of the two elements should result. If possible, the oxidizing agent should also convert any americium accompanying the plutonium to a sexivalent compound, but it should not introduce any substance which will precipitate with lanthanum and interfere in any subsequent method of determination. APPARATUS A N D REAGENTS
Spectrophotometer, Beckman Model DU, with 1-cm. Corex cells. Heating block, aluminum, 25 X 50 X 132 mm., with six holes, 14.5 X 42 mm., drilled into the 25 X 132 mm. side a t a 10' angle to the horizontal position for heating 12 X 100 mm. test tubes. The block should be heated with an adjustable hot plate, similar to a Fisher Autemp heater. Test tube, quartz, 12 X 100 mm. Plutonium metal, with average - assay
of 99.9%.
All other materials used in this investigation were reagent grade; solutions were prepared with double-distilled water. METHOD
Sample Preparation. Dissolve sam-
ples of plutonium metal or alloy in perchloric acid. Evaporate plutonium nitrate or chloride solutions in the presence of perchloric acid and convert t o perchlorate solutions. The volume of samples in dissolved form must be 5 ml. or less, or samples must be concentrated by evaporation before transfer to a quartz test tube. The sample should be selected to include 25 to 200 y of lanthanum and, preferably, no more than 50 mg. of plutonium. If a sample contains more than 200 y of lanthanum, the final solution of lanthanum 8-quinolinolate should be diluted to some suitable, known volume greater than 10 ml. before measuring the absorbance. If it is necessary to use samples containing as much as 100 mg. of plutonium, special care must be taken in dissolving the metal or alloy in perchloric acid to avoid too violent a reaction and formation of some in-
soluble plutonium dioxide. Also, a third lanthanum fluoride precipitation is advisable with large amounts of plutonium. Procedure. Transfer a sample to a quartz test tube (12 X 100 mm.). For metallic samples, place the roundbottomed end of the quartz tube under mater in a beaker to avoid an excessively rapid dissolving reaction. Add 0.4 ml. of 70% perchloric acid but keep the open end of the tube covered with a portion of Kleenex tissue to avoid any spread of radioactive spray during dissolving. If the sample is in dissolved form, add 0.4 ml. of 70% perchloric acid to the sample solution and carefully evaporate to a volume of about 0.5 ml. An infrared lamp may be used for this evaporation, but boiling and too rapid heating must be avoided. With either type of sample, transfer the quartz tube to the aluminum heating block and heat at 225' C. for 10 minutes. Perchloric acid fumes should be evolved during the last few minutes of heating. Remove the tube from the heating block to cool the oxidized sample to room temperature. Add 3 ml. of distilled water, rinsing down the inner wall of the tube during this addition. Dissolve all material by mixing rvith a platinum wire stirrer and leave the stirrer in the tube. Add dropwise 0.4 to 0.8 ml. of concentrated hydrofluoric acid to the platinum stirrer in the quartz tube If the acid is allowed to flow down the stirrer, direct contact between the reagent and quartz can be avoided. Stir the solution thoroughly and then rinse the stirrer with a feiv drops of LY hydrofluoric acid when it is removed from the tube. Allow the quartz tube to stand for 10 minutes and then centrifuge a t maximum speed for 5 minutes. Remove the tube from the centrifuge and, without disturbing the collected precipitate, dispose of all supernatant liquid by suction to a container for disposal of radioactive solutions. Invert the tube to collect the last portion of liquid by suction. Add 1 to 2 ml. of 1-Y hydrofluoric acid, stir the mixture of precipitate and wash liquid with the platinum stirrer, and again rinse the platinum Fire as it is withdrawn. Centrifuge for 5 minutes and again dicard the supernatant liquid by suction. Add 0.3 ml. of 70% perchloric acid to the precipitate in the quartz tube. Heat the tube in an aluminum heating block a t 225' C. for 7 or 8 minutes; acid fumes are evolved during the last 3 or 4 minutes. Remove the tube from
the heating block, and allow the tube to cool to room temperature. Rinse the inner surface of the tube with 0.5 ml. of distilled water and collect all liquid a t the bottom of the tube by centrifuging briefly. With a pipet, transfer the solution quantitatively to another quartz test tube (12 x 100 mm.) which is reserved for work with solutions containing no more than microgram quantities of plutonium. Complete transfer of the solution can be assured by using 3 0.7- or 0.8-ml. portions of distilled water to rinse the original tube. Add 100 mg. of solid ammonium persulfate and 2 drops of 0.05% thymol blue indicator solution to the quartz tube. Dissolve and mix with a platinum stirrer. Add 1.5N ammonium hydroxide dropwise, with stirring, until excess acid is nearly neutralized. Then complete the neutralization with 0.15N ammonium hydroxide. The indicator changes from red to orange or yelloworange. The solution volume should not exceed 5 ml. Rinse the stirrer with several drops of water as it is removed from the solution. Heat the quartz tube in an aluminum heating block a t 85" C. for 15 to 20 minutes. Then remove the tube from the heating block and replace the stirrer in the tube -4dd 0.4 ml. of concentrated hydrofluoric acid, having the reagent first touch the platinum stirrer to avoid direct contact with the quartz tube. Use the last several drops of acid to rinse the stirrer after mixing the solution thoroughly. Allow the tube to stand for 10 minutes and then centrifuge it for 5 minutes. Dispose of all supernatant liquid by suction, wash the precipitate with 1 to 2' ml. of IN hydrofluoric acid, centrifuge, and again remove all wash liquid by suction. Add 0.3 to 0.4 ml. of 70% perchloric acid: dissolve the precipitate and remove all fluoride by heating the tube in an aluminum heating block a t 225' C. for 7 or 8 minutes. Rinse the inner surface of the quartz tube with 1.5 to 2.0 ml. of water. Add 0.5 ml. of 1% 8-quinolinol in 2-41 acetic acid. Then add 1.5N ammonium hydroxide dropn-ise, with stirring, until the solution is neutralized. This point is reached when the intensity of the yellow solution increases and lanthanum precipitates. Add several drops of the same ammonium hydroxide solution and mix thoroughly. Rinse the stirrer with water in removing it from the tube. Heat the tube for 2 minutes in a steam bath, and then allow the precipitate to settle for 1 hour. Collect the precipitate on a 15-ml., medium porosity, fritted-glass, filter funnel which is supported by a micro bell jar. Wash the quartz tube and precipitate on the funnel thoroughly, but with a minimum of distilled water. Discard all filtrate and washings; wipe the lower end of the filter funnel with a Kleenex tissue. Place a 10-ml. volumetric flask under the filter funnel supported by the bell jar. Rinse the quartz tube with several milliliters of O . l N hydrochloric acid and add this rinse solution to the precipitate on the funnel. Carefully apply suction to the
bell jar to draw the dissolved precipitate into the volumetric flask without loss of solution. Rinse the funnel with 0.1N hydrochloric acid and transfer this rinse to the volumetric flask until the total volume is 8 or 9 ml. Dilute the solution in the flask to exactly 10 ml. with 0.1N hydrochloric acid and mix thoroughly. Measure the absorbance of this solution a t 365 m,u against a reference cell containing distilled water. Subtract any cell correction, and delermine the equivalent amount of lanthanum in micrograms for the corrected absorbance by reference to a calibration curve. Calibration Curve. Prepare a calibration curve by dissolving 50-mg. samples of high purity plutonium metal in 0.4 ml. of 70% perchloric acid in a quartz tube. Then add a known aliquot of a standard lanthanum nitrate solution to each dissolved metal sample. Follow the directions under Procedure for samples in dissolved form, beginning with a careful evaporation of the solution in the quartz tube to a volume of about 0.5 ml., and concluding with a plot of absorbance readings against micrograms of lanthanum. A standard lanthanum solution can be analyzed gravimetrically by transferring aliquots containing a t least 10 mg. of lanthanum to 100-ml. beakers and diluting to about 30 ml. Then heat to boiling, and add 5 ml. of 2.47 acetic acid and an excess of 3% 8quinolinol in ethvl alcohol. Add ammonium hvdroxide until no more precipitate is formed and a strong odor of ammonia is detected. After warming the solution for several minutes, allow it to stand for 1 hour before filtering on a tared, sintered-glass, filter crucible. Wash with warm water, dry a t 130' C., cool, and reweigh the crucible. The factor for lanthanum oxinate is 0.2433. RESULTS AND DISCUSSION
The results obtained with the proposed procedure in analyzing 15 solutions containing plutonium and varying amounts of lanthanum are shown in Table I . The average error for these determinations was 3 y of lanthanum. For these, and all other measurements which were made, the average molar absorbance index and the standard deviation were 5230 i. 170. Another set of 15 solutions containing amounts of lanthanum varying between 40 and 200 y was prepared, but without adding any plutonium. I n this case, precipitation with hydrofluoric acid was unnecessary and the lanthanum determination began with the addition of 8quinolinol. The reliability of the photometric measurements did not differ significantly from that of the measurements obtained after a plutoniumlanthanum separation (Table I). I n both sets of samples the standard deviation of a single measurement was in the range of 0.010 to 0.012 absorbance unit, equivalent to 3 y of lanthanum. Perchloric acid was selected as an
Table I. Lanthanum in PlutoniumLanthanum Solutions
Lanthanum, Added" Found
Absorb-
Error,
ance
Y
0 140 -2 0.195 0 267 0 329 -3 0.405 +7 0 428 -3 120 0 453 0 128 0 468 -4 130 0 505 7 4 149 0 579 +5 151 0 572 +I 157 0.610 $5 187 0 698 -1 ~209 0.794 +2 221 0 845 +4 a Each solution sample also contained 50 mg. of plutonium. 39 49 71 91 101 117
37 52 71 88 108 114 120 124 134 154 152 162 186 ~. 211 225
+;
oxidizing agent for the initial formation of sexivalent plutonium because this same reagent was also required for dissolving plutonium metal or assisting in the removal of nitric acid from a sample of dissolved plutonium nitrate. Oxidation with perchloric acid leaves no product which interferes with the precipitation of lanthanum fluoride. For this precipitation, the dissolved sample was adjusted to 1N with respect t o both hydrofluoric and perchloric acid. Under these conditions, and with as little as 25 y of lanthanum, there n-as no difficulty in obtaining quantitative separations after centrifuging unless some extraneous materials such as paper fibers were inadvertently added t o the precipitating solution. It was essential to have the lanthanum fluoride adhere strongly to the centrifuge tube and collect the last trace of solution from the inverted tube by suction for complete removal of plutonium from lanthanum in two successive precipitations. If necessary, the coagulation of lanthanum fluoride can be aided by the addition of acetate ion (3). During the investigation of fluoride separations, a combination of one oxidation and one precipitation step did not give the necessary degree of separation of plutonium from lanthanum. T K O successive oxidationprecipitation steps were essential. Substitution of ammonium persulfate for perchloric acid as an oxidizing agent in the second oxidation gave more consistent results. K i t h dissolved samples which contained significant quantities of sulfate, it was necessary to use ammonium persulfate for both oxidations. These sulfate-containing samples formed crystals of red plutonium(IT') sulfate during the fuming operation with perchloric acid and complete conversion to sexivalent plutonium became extremely slow. After dissolving the plutonium sulfate crystals in dilute acid, complete oxidation was VOL. 30, NO 10, OCTOBER 1958
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accomplished n ith ammonium persulfate. The investigation of interfering elements was restricted because the proposed procedure was designed for samples of relatively pure plutonium. Many elements remain in solution with plutonium(V1) when lanthanum is precipitated as the fluoride, but rare earths, cerium(IV), uranium(IV), americium(111) or (IV), thorium, and the alkaline earths will precipitate with lanthanum. For samples containing small amounts of thorium, it was convenient to determine this element in a separate sample ( 1 ) and correct the lanthanum results for thorium. The most successful separations of americium n-ere accomplished when ammonium persulfate n as em-
ployed in two successive oxidation-precipitation steps. There were some cases, however, where the unoxidized americium and plutonium accompanying the lanthanum amounted to as much as 2 or 3 y. These results emphasized the need for determining americium and plutonium in the final solution of lanthanum 8-quinolinolate by radioanalysis, if it is essential to correct the lanthanum determination for this low level of contamination. LITERATURE CITED
( 1 ) Bergstresser, K. S., Smith, 11. E.,
Los Alamos Scientific Laboratory, LA1839 (September 1954). (2) Hyde, E. K., in “The Actinide Elements,” G. T. Seaborg, J. J. Katz,
eds., 1st ed., Div. IV, Vol. 14-4,Chap 15, p. 576, National Nuclear Energy Series, RlcGraw-Hill, New York, 1954. (3) Popov, A. I., Knudson, G. E.,’ANAL. CHEX.26,892 (1954). (4) Rein, J. E., Los Alamos Scientific Laboratory, “A Modified Spectrochemical Method for Determination of Submicrogram Amounts of Certain Elements in Oxidized Plutonium Solutions,” in preparation. ( 5 ) Reinschreiber, J. E., Langhorst, Jr., A. L.. Elliott. $1. C.. Los illamos Scientific Laboratory, LA-1354 (February 1952). (6) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” 2nd ed., p. 509, Interscience, Kew York, 1950. RECEIVEDfor review January 21, 1958. Accepted May 12, 1958. Work performed under auspices of C . S. Atomic Energy Commission.
Cathodic Electrodeposition Methods for Cobalt DARNELL SALYER’ and THOMAS R. SWEET Mcfherson Chemical laboratory, The Ohio State Universify, Columbus I 0, Ohio
b
Radiotracer techniques and cobalt-
60 were used to study methods for the determination of cobalt by cathodic eledrodeposition. The quantity of cobalt left in the electrolytic solution was determined after electrodeposition by 10 of the most frequently used procedures for cobalt. Some cobali remained in solution in every case. The amount varied from 0.01 to 4.0 mg., with an average of 0.46 mg. of cobalt for 43 determinations. For the most part the cobalt left in solution is the result of fhe washing of the deposits and is not simply metal that was never deposited. A special washing procedure whereby the electrolytic solution was gradually replaced by distilled water until the current dropped to zero did not decrease the cobalt left in solution. The average positive error for 43 single determinations of cobalt was 0.74 mg. before, and 1.1 9 mg. after correcting for the cobalt left in solution. Correcting the data for the residual cobalt increased the errors.
T
most common difficulty encountered in the electroanalysis of cobalt is said to lie in the fact that special measures must be taken to deposit the last traces (8). The metal remaining in solution is generally determined gravimetrically (11) or colorimetrically (4, 6). It can be neglected for large cobalt samples (>0.25 gram), but beHE
1 Present Rome, Ga.
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address, Shorter ANALYTICAL CHEMISTRY
College,
comes increasingly important for smaller quantities. The aim of the present work was to examine some of the most frequently used methods for the electrolytic determination of cobalt. The use of radiocobalt and a solution counting technique provided a convenient method of determining the residual cobaltLe.. the cobalt left undeposited. REVIEW OF METHODS
Satisfactory results for the electroanalysis of cobalt have been reported when deposition is made from ammoniacal or from certain slightly acidic solutions. The presence of various salts in the bath is said to be beneficial. Depolarizers are used to prevent anodic deposits of cobalt(II1) oxide. Acidic solutions that have been used include those containing formate or lactate, reported by Smith (f6),and the phosphate bath described by Perkin and coworkers (IS, 14). Electrolytic procedures have been described which utilize ammoniacal solutions of ammonium acetate, formate or lactate (15), oxalate (9), chloride ( I 7 ) ,bifluoride (S), sulfate (f9), bisulfite (I), thiocyanate (2), and borate (5).
A review of most of the early work on
the electroanalysis of cobalt has been given by Watts (18). A recent publication on the subject is by Hague, Maczkowske, and Bright (6). The conditions for cobalt deposition in some of the most frequently used methods are tabulated in Table I.
APPARATUS A N D REAGENTS
An Eberbach rotating electrode electroanalyzer was used for the depositions. Ordinary cylindrical platinum gauze cathodes were used. They were 2 inches long by 11/2 inches in diameter and had 4-inch solid wire platinum stems; the surface area of each was approximately 100 sq. em. The platinum anode stirrers mere 1 X 3/4 inch plates M ith stems. The apparatus for radiation measurement consisted of a Model LC1 liquid counter apparatus (Nuclear Instrument and Chemical Corp., Chicago) and a Potter predetermined decade scaler, Model 341. I n addition to the Nuclear D52 tube provided with the solution counter set, a Tracerlab TGC-6 G l I tube was also used. Cobalt solutions were prepared from spectrographically pure cobalt sponge of Johnson RIatthey and Co., Ltd., London. Stock solutions containing about 1 mg. of cobalt per ml. as the sulfate were made active by the addition of 0.5 to 1 nic. of high purity cobalt60 per liter of solution. They were standardized by titrating with (ethylenedinitri1o)tetraacetic acid. High specific activity radiocobalt was obtained from the Oak Ridge n’ational Laboratory as cobalt(I1) chloride in hydrochloric acid solution. EXPERIMENTAL TECHNIQUES
Electrolytes. Using aliquots of the standard active cobalt solution (containing 50 t o 60 mg. of cobalt). electrolyte baths were prepared and the electrolyses were made as described in the method under investigation (Table I). The cathodes were removed, without interrupting the
